12.1.3 Chip Setup

When the chip is powered on, the system source clock is the Main RC oscillator. No external crystal or clock is needed during the external memories boot sequence.

The ROM code performs a low-level initialization that follows the steps described below:

  1. PLLA initialization
  2. Main system bus clock selection: when the PLLA is stabilized, the main system bus clock source is switched from the main clock to the PLLA clock. The PMC Status register is polled to wait for MCK Ready.

Once the clock configuration is done, the ROM code initializes the ROM code console interface and sends a RomBOOT character string on it.

For clock frequencies, see Table 12-14.

Note: The ROM code only seeks an external clock source before launching the SAM-BA Monitor to get a more accurate clock signal for USB.